1. Field of the Invention
The present invention relates to an electrostatic discharge (ESD) protection device and a method for producing the same. In particular, it relates to improvements of a discharge supporting electrode provided in an ESD protection device to promote electrostatic discharge.
2. Description of the Related Art
An overvoltage protection device which is of interest to the present invention is described in, for example, Japanese Unexamined Patent Application Publication No. 2008-85284 (Patent Document 1).
Patent Document 1 describes a material for an overvoltage protection device serving as a discharge supporting electrode for promoting discharge, the material containing a non-conductive powder (for example, silicon carbide with a particle diameter of 1 to 50 μm), a metal conductive powder (for example, copper with a particle diameter of 0.01 to 5 μm), and an adhesive (for example, a glass powder).
Patent Document 1 also describes a method for producing an overvoltage protection device, including a step of forming a material paste by evenly mixing a non-conductive powder, a metal conductive powder, and an adhesive at particular ratios, a step of applying the material paste onto a substrate by printing, and a step of subjecting the substrate to a firing treatment (temperature: 300° C. to 1200° C.).
However, according to the technology described in Patent Document 2 in which a glass powder is used as an adhesive, the glass component may become unevenly distributed depending on the dispersion state of the glass powder, possibly resulting in insufficient bonding between the metal conductive powder and the non-conductive powder. Accordingly, there is a problem in that shorting failure easily occurs especially in the event of high-voltage ESD.
An example of a technology that can address the aforementioned issue is one described in International Publication No. 2009/098944 pamphlet (Patent Document 2).
Patent Document 2 describes use of a discharge supporting electrode in which a conductive material (Cu powder or the like) coated with an inorganic material (Al2O3 or the like) is dispersed. According to the technology described in Patent Document 2, exposure of the conductive material is less compared to the technology described in Patent Document 1 and thus the insulation reliability can be enhanced. Moreover, since shorting between conductive materials rarely occurs even at an increased conductive material content, discharge can be promoted by increasing the amount of the conductive material and thus the peak voltage can be decreased.
However, the technology described in Patent Document 1 has the following challenges to be met.
The “conductive material coated with an inorganic material” according to the technology described in Patent Document 2 is merely a conductive material having surfaces coated with fine particles of an inorganic material, as described in paragraphs [0034] and [0094] and FIG. 4 of Patent Document 2. Accordingly, it is relatively difficult to completely cover the surfaces of the conductive material with an inorganic material and the conductive material may become exposed. Accordingly, further improvements have been desirable in relation with the insulation reliability.                Patent Document 1: Japanese Unexamined Patent Application Publication No. 2008-85284        Patent Document 2: International Publication No. 2009/098944 pamphlet        